In response to the development of various optical recording media in recent years, an optical pickup device that can be commercially used for recording or reproducing from two types of optical recording media has been known. For example, a device that can record or reproduce either one of a DVD (digital versatile disc) and a CD (compact disc including a CD-ROM, CD-R and CD-RW) using one optical pickup device has been used. For the DVD, it is known to use visible light with a wavelength of, for example, 657 nm, in order to increase the density of the recording. However, there are some CDs that have no sensitivity to visible light, and thus near-infrared light with a wavelength of approximately 790 nm is commonly used.
Therefore, in an optical pickup device that is commonly used for both of these optical recording media, a so-called double wavelength beam system using two beams of different wavelengths may be used to irradiate a recording medium. For the different recording media, different numerical apertures of the objective lens based on different technical standards are used. For example, for the DVD technical standard, the numerical aperture is set at 0.6, and for the CD technical standard, the numerical aperture is set approximately in the range of 0.45 to 0.52. In addition, the substrate thickness of the recording media, with the substrate being a protective layer generally made of polycarbonate (PC), is standardized to different values depending on the recording medium, such as 0.6 mm for a DVD and 1.2 mm for a CD.
As described above, since the substrate thickness of the optical recording medium is standardized and differs according to the type of optical recording medium, the amount of spherical aberration introduced by the substrate is different based on the different standardized thickness of the substrate of different recording media. Consequently, for optimum focus of each of the light beams on the corresponding optical recording medium, it is necessary to optimize the amount of spherical aberration in each light beam at each wavelength for recording and reproducing. This makes it necessary to design the objective lens with different focusing effects according to the light beam and recording medium being used.
Additionally, in response to rapid, almost daily, increases of data capacity, the demand for an increase in the recording capacity of recording media has been strong. It is known that the recording capacity of an optical recording medium can be increased by using light of a shorter wavelength and by increasing the numerical aperture (NA) of an objective lens. Concerning a shorter wavelength, the development of a semiconductor laser with a shorter wavelength using a GaN substrate (for example, a semiconductor laser that emits a laser beam of 408 nm wavelength) has advanced to the point where this wavelength is now available for use.
With the development of short wavelength semiconductor lasers, research and development of AODs (Advanced Optical Disks), also known as HD-DVDs, that provide approximately 20 GB of data storage on a single side of an optical disk by using short wavelength light is also progressing. As the AOD standard, the numerical aperture and disk thickness are selected to be close to, but slightly different, from those of DVDs, with the numerical aperture (NA) and disk thickness for an AOD being set at 0.65 and 0.6 mm, respectively.
Furthermore, research and development of Blu-ray disk (BD) systems that use a shorter wavelength of disk illuminating light, similar to AOD systems, has progressed, and the standardized values of numerical aperture and disk thickness for these systems are completely different from the corresponding DVD and CD values, with a numerical aperture (NA) of 0.85 and a disk thickness of 0.1 mm being standard. Unless otherwise indicated, hereinafter, AODs and Blu-ray disks collectively will be referred to as “AODs.”
The development of an optical pickup device that can be used for three different types of optical recording media, such as AODs, DVDs and CDs as described above, has been demanded and objective optical systems for mounting in such devices have already been proposed. For example, an objective optical system that includes a diffractive optical element with a refractive surface and a diffractive surface and a biconvex lens is described on page 1250 of Extended Abstracts, 50th Japan Society of Applied Physics and Related Societies (March, 2003). The objective optical system described in this publication is designed so that: second-order diffracted light from the diffractive optical element is used for a BD optical recording medium; first-order diffracted light from the diffractive optical element is used for a DVD optical recording medium; and also first-order diffracted light from the diffractive optical element is used for a CD optical recording medium. The rear surface of the diffractive optical element (the side opposite the illuminating light beams), is concave in order to aid in correcting spherical aberration that is created by the thickness of the protective layer of each optical recording medium. The spherical aberration created varies with the thickness of the protective layer. Chromatic aberration is also improved relative to a single component lens.
In the technology described in the above-mentioned publication, in order to reduce the generation of coma associated with a shift of the objective optical system relative to an incident luminous flux, when recording or reproducing information to/from the BD, the design is such that the light incident on the diffractive optical element is converging light. Further, when recording or reproducing information to/from the DVD or the CD, the design is such that the light incident on the diffractive optical element is collimated light and diverging light, respectively.
However, there presently is strong demand for a compact device that provides greater freedom in positioning the objective optical system within the recording and reproducing device. In order to achieve this, it is necessary to create a design such that collimated light, rather than diverging or converging light, be incident on the objective optical system for at least two of the three light beams that are being used. Additionally, in particular with regard to the light beam with the shortest wavelength of the three light beams, if converging light is incident on the diffractive optical element, there are problems of the diffraction efficiency being reduced due to the angle of incidence of the light rays on the diffractive grooves of the diffractive optical element being tilted from the desired angle of incidence, which greatly decreases the stability of the tracking.